In general, the lithography process used during semiconductor fabrication comprises the following steps: A layer of photoresist (PR) material is first applied on the surface of the wafer. The resist layer is then selectively exposed to radiation, such as ultraviolet light, electrons, or X-rays, with the exposed areas defined by a mask.
After exposure, the PR layer is subjected to development which alters the chemical property of the PR being exposed in the unwanted areas of the PR layer, exposing the corresponding areas of the underlying layer. Depending on the resist type, the development stage may destroy either the exposed or unexposed areas of the PR layer. The areas with no resist material left on top of them are then being processed to form patterns through additive and/or subtractive processes, allowing the selective deposition or removal of material on the substrate (or other base layer).
During development, the unwanted areas in the PR are dissolved by the developer. In the case where the exposed areas become soluble in the developer, a positive image of the mask pattern is produced on the resist. Such a resist is therefore called a positive photoresist. Negative photoresist layers result in negative images of the mask pattern, wherein the unexposed areas are soluble in the developer and those exposed areas are made non-soluble or significantly less soluble in the developer. Wafer fabrication may employ both positive and negative photoresists, although positive resists are preferred because they offer higher resolution capabilities. Since wafer fabrication may employ both positive and negative photoresists, it is therefore desirable, in certain circumstances, to have a cost-effective way to make a reverse-image of a mask.